The present invention relates to a method for erasing a radiation image remaining in a stimulable phosphor sheet employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed, and has been practically employed. The radiation image recording and reproducing method employs a stimulable phosphor sheet (i.e., radiation image storage panel) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as xe2x80x9cstimulating raysxe2x80x9d) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The sheet thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly employed.
In the above method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resource and economic efficiency because the stimulable phosphor sheet can be repeatedly used while the radiographic film is consumed for each radiographic process in the conventional radiography.
The stimulable phosphor sheet has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. The phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the layer comprising agglomerated phosphor soaked with a polymer is also known. In any types of phosphor layers, the stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the stimulable phosphor sheet in an amount proportional to the applied radiation dose, and a radiation image of the object is produced in the sheet in the form of a latent image (i.e., radiation energy-stored image). The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the phosphor sheet with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The radiation image recording and reproducing method can be performed by means of an all-in-one type apparatus comprising recording means (by which a radiation image is recorded on the phosphor sheet), reading means (by which the image recorded in the phosphor sheet is read by the steps of exciting the stimulable phosphor with stimulating rays to release stimulated emission and photoelectrically detecting the emission), erasing means (by which the radiation image remaining in the phosphor sheet is erased with an erasing light), and conveying systems connecting each means for conveying the phosphor sheet. The above-mentioned means may be separated into two apparatuses, namely a recording apparatus comprising the recording means and a reading apparatus comprising the reading means and the erasing means.
It is preferred that the radiation image recording and reproducing method shows a high sensitivity and gives a radiation image of good quality such as high sharpness and good graininess.
The erasing step is performed to prevent a radiation image to be produced in the next run of the radiation image recording and reproducing procedure from deterioration which is caused by radiation energy still remaining in the phosphor sheet after the radiation image reading procedure is complete, and/or radiation energy brought by radioisotope contaminants in the phosphor sheet or environmental radioactive substance.
The erasing step should be performed as efficiently as possible. In more detail, the erasing step should be performed in a period of time as shorter as possible, using a smaller amount of energy (as to consumed electric power). Moreover, the stimulable phosphor sheet having been subjected to the erasing step should have a small value in the erasure value which is defined in terms of a value of (amount of stimulated emission after the erasing step)/(amount of stimulated emission before the erasing step), should give little reappearance of the previously formed radiation image, and should show little fogging under exposure to ultraviolet rays.
In order to satisfy the above-mentioned requirements, various erasing methods have been heretofore proposed. For instance, Japanese Patent Publication (examined) H1-59566 discloses the use of a stimulable phosphor sheet containing a visible to infrared light-absorbing stimulable phosphor in combination with an optical filter for cutting a light of a shorter wavelength in the erasing procedure so that the phosphor sheet can be free from UV fogging (which means absorbing a light of a wavelength region shorter than its stimulating wavelength region). Japanese Patent publication (examined) H3-79696 discloses the use of an erasing light source such as a white light fluorescent lamp which emits a light in the wavelength region of 400 to 600 nm but does not emit a light in the infrared region. Japanese Patent Provisional Publication 63-97939 describes employment of a fluorescent lamp and a ultraviolet ray-removing filter (i.e., filter absorbing more than 80% of a light of a wavelength of shorter than 440 nm) in combination.
Each of Japanese Patent Provisional Publications H4-1746, H5-119412 and H5-216142 describes a procedure comprising a first erasing step of applying a light which contains a light in the ultraviolet region (200-400 nm) and a second erasing step of applying a light which contains no light of the ultraviolet region, so as to remove a residual radiation image comprising electrons not only at a shallow level but also at a deep level. Japanese Patent Provisional Publication H4-156533 describes a procedure comprising a first erasing step of applying a light of approximately 400 nm and a longer wavelength region (for prevention of UV fogging) and a second erasing step of applying a light containing no light of approximately 600 nm and a shorter wavelength region.
The present inventor has noted that a stimulable phosphor sheet containing a halogen atom-containing stimulable phosphor such as a stimulable europium or cerium activated alkaline earth metal halide phosphor gradually lowers in its sensitivity when the erasing procedure employing a light containing a light of a ultraviolet region (which is an effective erasing procedure) is repeatedly applied to the stimulable phosphor sheet.
Accordingly, it is an object of the invention to provide a method for efficiently erasing a residual radiation image which remains in a stimulable phosphor sheet having been subjected to the reading procedure, without lowering of the sensitivity.
As a result of further study, the inventor has discovered that the lowering of sensitivity of the stimulable phosphor sheet is caused by coloring (i.e., yellowing) of the phosphor sheet with halogen released from the stimulable phosphor crystals. The release of halogen is caused by accumulation of energy of the applied ultraviolet rays. The inventor has continued the study, and found that the lowering of sensitivity caused by the deterioration of the stimulable phosphor crystals can be effectively prevented by removing a light of a shorter wavelength region from the erasing light in the ultraviolet to visible region, particularly, by removing line spectrum of mercury in the wavelength region of shorter than 370 nm when a fluorescent lamp is employed as the source of erasing light.
The present invention resides in a method for erasing a radiation image remaining in a stimulable phosphor sheet in which a radiation image was recorded and then read by applying stimulating rays onto the phosphor sheet and collecting stimulated emission from the phosphor sheet, comprising a step of applying onto the stimulable phosphor sheet an erasing light containing essentially no light of a wavelength of shorter than 370 nm.
Preferred embodiments of the invention are described below.
(1) The erasing light contains a light of a wavelength region of 370 to 500 nm.
(2) The erasing method comprises a further step (i.e., second erasing step) applying onto the phosphor sheet an erasing light containing essentially no light of a wavelength of shorter than 500 nm.
(3) The erasing light is emitted by a fluorescent lamp and then transmitted through a ultraviolet ray-removing filter.
(4) The erasing light is emitted by a light source emitting neither a light of ultraviolet region nor a light of infrared region.
(5) The erasing light in the further step (i.e., second erasing step) and the erasing light in the previous step (i.e., first erasing step) are applied to the phosphor sheet in a ratio of light amount of 15/85 to 45/55.
(6) The stimulable phosphor sheet contains a stimulable halogen-containing phosphor.
(7) The stimulable phosphor sheet contains a stimulable europium or cerium activated alkaline earth metal halide phosphor.